Fixes the condition for the attestation for content protection.

During the debug of my CL crrev.com/23783013, I noticed that
the value for kAttestationForContentProtection setting is
- true by default
- if it sets to false, the stored value becomes true

And noticed this conditions, if content_protection_enabled() is
set to true, the stored value becomes false.

BUG=NONE
R=mnissler@chromium.org

Review URL: https://chromiumcodereview.appspot.com/24361005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225129 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225129